Recently, along with enhancement of the operating speed of personal computers, the spread of network infrastructure, and increased and lower-priced mass storage, data of documents or image, which were conventionally printed on paper, can be received simply as electronic information so that opportunities to read such electronic information have increased dramatically.
There were used, as a means for reading electronic information, conventional liquid crystal displays or CRTs and recent emission type displays, such as organic electroluminescence displays. Specifically, when electronic data is document data, it is necessary to notice this reading means over a relatively long period of time. It is hard to say that such an action is a kindly means to humans. There are generally known disadvantages of emission type displays such that flickering tires human eyes, they are awkward to carry about, the reading posture is restricted, it is necessitated to gaze at a stationary picture plane, and electric power consumption increases when reading over a long time.
As a display means to redeem the foregoing disadvantages is known a (memory type) reflective display which employs external light and does not consume electrical power for image retention. However, based on the reasons below, it is hard to say that such displays provide sufficient performance.
For instance, a system using a polarizing plate such as a reflective liquid crystal display exhibits a relatively low reflectance of up to 40%, resulting in difficulty in displaying whiteness and methods of preparing constituent members are not necessarily simple. A polymer dispersed liquid crystal display requires a relatively high voltage and employment of the difference in refractive index between organic compounds does not result in images with sufficient contrast. A polymer networked liquid crystal display has problems such that it requires a relatively high voltage and a complex TFT circuit to enhance memory. An electrophoretic display element needs relatively high voltage of more than 10 V, and there is a concern of durability of the electrophoretic particles, due to their tendency to coagulate.
There are known, as a display system to overcome these disadvantages of the foregoing systems, an electrochromic display element (hereinafter, denoted as EC system), and an electrodeposition (hereinafter, denoted as ED system) system utilizing dissolution and deposition of metals or metal salts. The EC system enabling full color display at a low voltage of not more than 3 V exhibits advantages such as simple cell configuration and excellent while color quality. The ED system, which can be driven at a relatively low voltage of not more than 3 V, also exhibits advantages such as simple cell configuration and being superior in black and white contrast as well as in black color quality. There are disclosed various methods (refer to Patent Documents 1-5, for example).
After considerable effort during detailed studies concerning techniques disclosed in each of the above-described Patent Documents, the inventors have found out that stability in reflectance during repeatedly driving causes a problem in the case of conventional techniques. As the means to solve this problem, provided are a method of adding a ferrocene series compound into an electrolyte as a redox buffering agent (refer to Patent Document 6, for example), a method of fixing a redox promoter or the like on an electrode (refer to Patent Document 7, for example) and a method in which a radical polymer is utilized (refer to Patent Document 6, for example), but in order to meet high requirement specification desired from users in recent years, further improvement should be made.
Patent Document 1: WO No. 2004/068231
Patent Document 2: WO No. 2004/067673
Patent Document 3: U.S. Pat. No. 4,240,716
Patent Document 4: Japanese Patent No. 3428603
Patent Document 5: Japanese Patent Open to Public Inspection (O.P.I.) Publication No. 2003-241227
Patent Document 6: Published Japanese Translation of PCT International Publication No. 2007-508587
Patent Document 7: U.S. Pat. No. 7,253,940
Patent Document 8: Japanese Patent O.P.I. Publication No. 2007-298713